1. Field of the Invention
This invention relates to a process for the preparation of cross-linked maleic anhydride copolymers and more particularly, to a process for the cross-linking of an alkyl vinyl ether/maleic anhydride copolymer with a polyol compound.
2. The Prior Art
Lower alkyl vinyl ether/maleic anhydride (AVE/MA) copolymers, for example a copolymer of maleic anhydride with a C.sub.1 -C.sub.4 alkyl vinyl ether, such as methyl or ethyl vinyl ether, are known to the art and have been long recognized as water soluble ingredients for use in personal care, agricultural, medical and detergent compositions.
In particular, a methyl vinyl ether/maleic anhydride copolymer, commercially available under the trademark "Gantrez" is disclosed in U.S. Pat. Nos. 5,202,112 and 5,334,375, as a thickener and antibacterial-enhancing agent for noncationic antibacterial agents used in oral care compositions such as toothpastes and mouthwashes. Gantrez is a methyl vinyl ether/maleic anhydride copolymer having an average molecular weight (M.W.) of about 30,000 to about 1,000,000 available from ISP Investments, Inc. Gantrez S-97 pharmaceutical grade (M.W. 70,000) is preferred for use in oral compositions.
It is known to the art, e.g. U.S. Pat. No. 5,385,729, that an AVE/MA copolymer having linear viscoelastic properties useful as a thickener in oral care compositions may be obtained by cross-linking the copolymer. Cross-linking the copolymer is carried out during the polymerization of the AVE and MA monomers by including in the reaction medium a preferably moderate, 3 to 15% by weight, amount of an unsaturated cross-linking agent, typically a C.sub.4 -C.sub.30 hydrocarbon containing an ethylenically unsaturated group in a non-conjugated, terminal relationship such as 1,7-octadiene, 1,9-decadiene, divinyl glycol, propylene glycol, butanediol, hexanediol and dodecanediol diacrylate.
U.S. Pat. No. 5,385,729 also discloses that the AVE/MA copolymer can be cross-linked by a post polymerization reaction, wherein polyfunctional cross-linking agents reactive with pendant carboxyl groups along the AVE/MA copolymer chain are used. Illustrative cross-linking agents are polyol compounds containing about 4 to about 30 carbon atoms including linear and cyclic polyols such as butane and octadecane diols, polyethylene glycol, glycerol, sucrose and pentaerythritol. The patentee teaches that in such post-polymerization cross-linking process, the anhydride ring of the AVE/MA copolymer must first be opened by hydrolysis to release free carboxy (COOH) groups needed for reaction with the polyol cross-linking agent before the cross-linking reaction can be accomplished. The patentee further teaches that post-polymerization cross-linking of the AVE/MA copolymer is less preferred than cross-linking with ethylenically unsaturated hydrocarbons, as the polyol cross-linking product tends to more easily be subject to hydrolysis with the resulting loss of the desired linear viscoelastic properties.